In an anti-lock control apparatus designed to prevent the locking of vehicle wheels at the time of braking the vehicle so as to ensure the steerability and running stability of the vehicle and to shorten the braking distance, the control modes of the brake fluid pressure (i.e., a pressure increasing mode, a pressure decreasing mode and a pressure holding mode) are determined in accordance with electrical signals from speed sensors for sensing the speeds of the respective vehicle wheels. In accordance with such pressure control mode, hold valves and decay valves both of a solenoid valve selectively open and close so as to increase, hold or reduce the brake fluid pressure. Such brake fluid pressure control is effected under the control of a microcomputer.
There is known a brake fluid pressure control apparatus of the type in which a fluid pressure control portion (modulator) for effecting the above-mentioned anti-lock control is incorporated in a master cylinder. In such an apparatus as disclosed in U.S. Pat. No. 4,641,895, the master cylinder comprises a primary piston and a secondary piston arranged in series therewith, which is known as of the tandem type, and brake fluid pressures in two brake lines of the brake device of the dual circuit type are controlled by the primary and secondary pistons, respectively. Hold valves of the normally open type are provided respectively in fluid passages connecting two fluid pressure chambers respectively to wheel cylinders, the fluid pressures in the two fluid pressure chambers being controlled by the primary and secondary pistons, respectively. Decay valves of the normally closed type are provided in fluid passages connecting the wheel cylinders respectively to a reservoir.
With such a construction, when a brake pedal is pressed down, the fluid pressures in the two fluid pressure chambers of the master cylinder are supplied to the respective wheel cylinders through the respective hold valves to increase the fluid pressures in the wheel cylinders, thereby applying a braking force to the wheels. When the anti-lock control starts to be operated, the hold valves close to hold or maintain the brake fluid pressures. Further, when the decay valves open, with the hold valves kept closing, the brake fluids in the wheel cylinders is relieved to the reservoir through the decay valves, so that the fluid pressures in the wheel cylinders are reduced, thereby reducing the braking force. Also, in the pressure increasing mode of the anti-lock control, the hold valves are operated into the open condition, and the pistons are operated by the brake fluid of high pressure supplied from a fluid pressure source such as an accumulator to thereby increase the fluid pressures in the wheel cylinders.
However, if any malfunction should develop in the fluid pressure source system, e.g., a fluid pressure pump or an accumulator, for example, the accumulator leaks a fluid pressure contained therein, during the anti-lock control, the braking force becomes inadequate. To compensate for this, the brake pedal has to be further pressed down, so that the brake pedal can not be still further be pressed down adequately (that is, there is no sufficient room for further pressing-down).
Further, in the case where in addition to the above-mentioned anti-lock control, the above brake fluid pressure control apparatus is designed to perform a traction control in such a manner that the brake fluid of high pressure from the fluid pressure source acts on the pistons as in the anti-lock control, the braking force becomes excessive. This not only imparts vibration to the vehicle body to provide an uncomfortable ride feeling but also to apply an undesirable load to the drive system of the vehicle.